This invention relates to a manufacturing method of brazing uniform plate/plate and plate/fin multi-channeled structures using an amorphous brazing foil as a brazing filler metal.
Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the base metal parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong and preferably corrosion resistant joint is formed.
One class of products produced by brazing processes is the heat exchanger, a three-dimensional structure comprised of alternating metal flat plates and fins or corrugated plates kept in tight, physical contact. Brazed joints mechanically connect and seal the contact areas between the flat plates and fins, as in the case of plate/fin heat exchangers or between the stamped corrugations in the plate/plate case. The corrugation profiles may have a chevron pattern, pressed-out indentations of various circular forms or some other profiles. In the brazed state these indentations are joined with flat plates or with each other forming an elaborate system of channels or interlocking cavities. In service, hot and cool liquids and/or gasses flow separately in these channels exchanging heat. In many cases, these structures are made from heat and corrosion resistant steels and operate at high temperatures as coolers in utility systems, as heat exchangers in aerospace structures, and as recuperators in chemical, food and other process industries.
The majority of multi-channeled brazed structures are produced using a filler metal placed between the base metal parts prior to the actual brazing process. Filler metal in powder form can be sprayed onto the surfaces of the base metal parts or applied in the form of a powder/polymer composite paste. In either case, filler metal in powder form is porous and contains considerable impurities in the form of oxides. The use of powder filler metals in this manner results in uneven, porous and poor quality joints. Alternately, filler metal in foil form can be placed directly between the base metal parts to be joined. Foil, by comparison, is 100% dense, carries fewer impurities and can be more accurately metered in the joint area. The use of foil in constrained assemblies, while being much more effective than powder, necessitates small variations in the foil thickness. This is particularly important when these assembled alternating base metal plates and foil preforms are constrained from mutual movement during brazing.
To optimize the brazed structure performance, one needs to conduct tedious, preliminary experiments to determine the proper amount of powder or foil relative to the base metal plate thickness and geometry. Moreover, constrained assemblies require that all parts have very precise dimensions and a very accurate part placement that is difficult and expensive to satisfy using the existing technology. To illustrate, consider corrugated sheet in a plate/plate heat exchanger that is 250 mm wide by 100 mm long by 0.1 mm thick, with a channel height of 5 mm. An optimized brazed joint will have a gap of 0.025 to 0.050 mm. A 1% variation in the channel height will cause the gap to change by 0.05 mm. A 1% deflection in the flatness of the sheet will cause the gap to change by 1 mm. The only way to seal these local large gaps is to fill them with brazing filler metal. When the gaps are large, but the amount of available filler metal is small or the filler metal has poor flow, then filling of the excessive gaps may not be sufficient in a mechanically constrained assembly of plates and preforms. As a result, there may be large unbrazed areas.
A properly designed heat exchanger must contain the liquids and/or gases in their appropriate channels and must safely withstand the pressure exerted upon it by the fluid media. These design criteria apply to each brazed joint in the stricture. The joint strength is a parameter determined by the joint size and microstructure. It is affected by the time-temperature brazing conditions. Given the larger number of joints in a heat exchanger, joint strength and integrity are rather difficult to predict and, even more, difficult to regulate. In the ideal case of high strength joint, a potential failure of the brazed structure under the critical internal pressure would occur in the structural parts made of the base metal rather than in the brazed joint.
Steel heat exchangers are typically brazed with Cu, Ni- or Co-based filler metals. Cu filler metal is available in foil form. The use of Cu, however, is limited to heat exchangers that experience moderate temperature and contain minimally corrosive media. Ni- and Co-based filler metals produce brazed joints capable of withstanding high temperatures and moderately corrosive media. The majority of Ni- and Co-based advanced filler metals that can be used for joining these structures contain a substantial amount of metalloid elements such as boron, silicon and/or phosphorus. Consequently, such alloys are very brittle in conventional crystalline form and available only as powders, powder-binder pastes and tapes and bulky cast preforms. Powders and powder-based preforms do not easily permit brazing of complex forms. However, these Ni- and Co-based alloys can be transformed into a ductile, flexible foil that is produced utilizing rapid solidification technology and which has an amorphous structure in the solid state. Such amorphous alloys for brazing applications are disclosed in many patents, for example U.S. Pat. Nos. 4,148,973 and 4,745,037. In spite of substantial advantages of rapid solidification technology achieved so far, the foil thus produced has cross-sectional and longitudinal thickness variations, sometimes exceeding xc2x140%.
Thus, there is a continuing need for an improved method of brazing complex three-dimensional plate/plate and plate/fin structures that can provide strong joints with controlled cross-section dimensions without being overly dependent on: (a) brazing foil thickness and its variations; and (b) the shape and accuracy of dimensions of fins and profiles.
There exists experimental data showing the beneficial effect of load applied normal to the joints of specimens subjected to brazing operations. In each of these specimens, amorphous metal foil was used as the brazing filler metal. The brazed joint thickness varied with the applied load. This data indicates the importance of load in the improvement of liquid filler metal wetting of rough gap surfaces and formation of non-porous brazes. Moreover, the self-adjusting interplay between the surface tension of a liquid filler metal and the applied load also optimizes the thickness, the microstructure and, most importantly, the strength of the brazement. This fundamental load effect provides the scientific basis for the proposed method of the present invention to improve brazed multi-channeled structures.
This invention is embodied in a brazing method comprising the steps of interposing an interlayer in an amorphous foil form between plates and fins to be joined, assembling parts in an unconstrained stack, applying a controlled load on the top of the stack, heating the assembly under suitable conditions to a temperature at which the interlayer melts and reacts with the base metal parts, and cooling the assembly to produce a structure with uniform joints having optimal dimensions, shape and strength.
The invention also comprises a brazed structure produced by the method described hereinabove.